Macrocyclic polyester oligomers (macrocyclic oligoesters, MPOs) have unique physical properties that facilitate the manufacture of polyester products. MPOs demonstrate certain processing advantages of thermosets, yet can be polymerized to form thermoplastic polyesters which provide superior toughness, excellent chemical resistance, high heat resistance, and are thermoformable, paintable, bondable, weldable, and recyclable. For example, MPO resins are available as easy-to-handle solid pellets that melt into a low viscosity fluid when heated. The low melt viscosity allows the MPO resin to easily fill molds or permeate fabrics to make prepregs. Furthermore, certain MPOs melt and polymerize at temperatures well below the melting point of the resulting polymer. Upon melting and in the presence of an appropriate catalyst, polymerization and crystallization can occur virtually isothermally, without significant heat generation and without production of volatile organic compounds (VOCs) or other harmful emissions. The polymerized product can be released without cooling the mold, and the time and expense required to thermally cycle a tool is favorably reduced.
MPO can be polymerized to form a thermoplastic polyester via ring-opening polymerization at elevated temperature in the presence of an appropriate catalyst. A block copolymer having a “hard block” and a “soft block” can be formed, for example, by reacting an MPO such as cyclic poly(butylene terephthalate) (cPBT) with a dihydroxyl-functionalized polymer in a polycondensation process, as described in co-owned U.S. Pat. No. 6,436,549 by Wang, the text of which is incorporated by reference herein in its entirety. Such block copolymers are useful, for example, in the manufacture of automotive body panels and chassis components, as well as aircraft wing skins.
However, commercial manufacture of block copolymer via polycondensation requires high capital investment in polycondensation reactors and associated process equipment. Most polycondensation systems are designed for high volume production of a single product, making it difficult to adapt such systems for low volume production, or for production of multiple products with fast changeover.
Furthermore, the manufacture of block copolymer via polycondensation can result in copolymer of insufficient molecular weight, particularly when using high loading levels of soft block reactant, such as dihydroxyl-functionalized polymer. This is likely due to the high concentration of species that act as chain stoppers in the polymerization reaction. Copolymers of higher molecular weight are desired because of their advantageous physical properties. For example, higher molecular weight copolymers generally exhibit increased strength. They also exhibit higher intrinsic viscosities and higher melt strength, and are therefore capable of being processed less expensively.
Moreover, the types and molecular weights of soft block reactants that can be used to produce block copolymers in polycondensation processes is limited due to miscibility problems. The soft block reactant (i.e. a polyol) must typically have molecular weight (Mw) less than 1000 g/mol to inhibit phase separation during the polymerization reaction. Most commercially-available dihydroxyl functionalized polymers have molecular weight below 1000 g/mol for this reason. Also, the weight percentage of the “soft block” component in the block copolymer must typically be kept low in order to sufficiently build up overall molecular weight of the block copolymer. However, this results in a block copolymer with overly-long hard block units. Moreover, highly polar hydroxyl-functionalized polymers are generally unusable in traditional polycondensation processes due to the increased miscibility problems they cause.
There is a need for methods of producing block copolymers whose structures and compositions can be better customized for a wider range of uses. For example, there is a need for manufacturing methods that produce block copolymers with desired physical and chemical properties such as chemical resistance, heat resistance, and high melting temperature (i.e. provided by the hard block), as well as toughness, hysteresis, and low modulus (i.e. provided by the soft block). In particular, methods are needed for manufacturing block copolymers with higher overall soft block content, as well as block copolymers with more highly polar soft blocks. Traditional polycondensation processes are limited in the type and amount of soft block component that can be incorporated into a block copolymer.
Furthermore, traditional polycondensation processes require high capital investment and are generally only designed for making one particular product in large quantities. Thus, there is also a need for block copolymer production methods that require lower capital investment, that can be adapted for high yields on small volume runs, and that allow for fast product changeover.